I cheat when it comes to simple keypad applications. I just take the PCB from an old USB keyboard and hack the 2 connectors . The ones I have used so far use 2 connectors, then I map the connectors into a table.

The nice thing is that the USB communication is already complete and Windows recognizes yor device without problem. You can connect more than 1 identical keyboards in Windows XP.

There are several examples on google to hack a USB keyboard.